An electronic module comprises one or more electronic components that are associated with a multilayer PCB (printed circuit board) interconnect circuit, which itself includes pads that are intended to receive balls of solder for the external electrical connection of the electronic module.
These components may be integrated within the PCB by being placed on an internal layer of the PCB, this being known as a PCB with embedded die. They may also be encapsulated within an epoxy resin after having been bonded and wired to the multilayer PCB circuit on the face opposite that which includes the solder balls; this is known as a ball grid array package.
The electronic components may be active components such as chips and/or passive components such as capacitors, inductors, etc., and/or MEMS (microelectromechanical systems).
The electronic module obtained may be standalone or may itself be added along with other electronic modules to a printed circuit including electrically conductive pads and a step of remelting the balls of the PCB circuit of the module is carried out in order to connect these balls to the pads of the printed circuit.
FIG. 1 shows a cross section of a ball grid array (BGA) package 1 that is intended to be added to a printed circuit; only two balls 4 have been shown on the pads 3 of the PCB circuit 2. The electronic component 32, in this instance a chip, is attached to the PCB circuit 2 by means of an adhesive bond or a preform 35. The electrical interconnect between the chip and the PCB circuit is formed by wires 31 that are soldered to the pads 33 of the chip and the pads 34 of the PCB circuit. The chip 32 and its connections 31 are moulded within a resin 10.
It is known that the resin within which the electronic components are moulded is watertight but not airtight (hermetic), since no organic material is airtight. This means that moisture (gas) will diffuse into the interior thereof, with consequences that are well known to users of plastic packages the world over:                The penetration of moisture into the interior of the package before it is added to the surface of the printed circuit produces an internal vapour pressure inside this package during the step of remelting the balls, which takes place at a maximum temperature of 260° C. This phenomenon is illustrated in FIG. 2, in which it is possible to see a non-hermetic ball grid array package 1 (the component is not shown so as not to overload the figure) which, exposed to atmospheric air, receives gaseous H2O by diffusion P1. The resulting pressure P2 will increase over a few seconds from atmospheric pressure to around twice that (273+260/273=1.95 Kg/cm2); this pressure produces internal stresses, the energy of which is dissipated by forming internal cracks in the package.        Since the penetration of water vapour takes place in the presence of acidic (Cl, SO4, etc.) or basic (Na, K, etc.) ions, acids or bases are formed which may attack the metal portions of the chips and in particular the aluminium forming the interconnect pads, which is an amphoteric metal.        
The international standardization body JEDEC has therefore determined multiple quality levels for these packages based solely on the criterion of moisture uptake before mounting on printed circuits. There are seven moisture sensitivity levels (MSL). Level 6 concerns the packages that are most sensitive to moisture: after a duration of six hours before mounting, such a package without protective packaging must be degassed again. Level 0 is the best (→the package may be stored for an indeterminate duration), but almost no plastic package achieve this level of performance.
Previous work has been carried out with the aim of overcoming the fact that organic materials are intrinsically non-hermetic. They are based on the deposition of a diffusion barrier made of a material that is non-organic and non-electrically conductive in order to avoid short circuits.
Any non-organic and non-electrically conductive material, such as oxides (Al2O3, etc.), nitrides (Si3N4, etc.), carbides, etc. can be used as long as they can be deposited at low temperature (lower than 100° C. for example) and are sufficiently deformable (ductile) so as not to crack after multiple thermal cycles. Silicon oxide (SiO2), and more specifically a non-stoichiometric oxide such as SiOx where x is smaller than 2, may also be mentioned. This inorganic layer may be deposited by plasma-enhanced chemical vapour deposition (PE-CVD) but the temperatures can generally reach 200° C., which is too high for plastic packages. Most fortunately, a technique referred to as the atmospheric plasma technique makes it possible to carry out these deposition operations below 100° C., which is more acceptable for this type of package.
Multiple patents have been filed:                “Method and device for hermetic encapsulation of electronic components” filed on 24 Aug. 1990 under No: 90 10631.        “Method and device for hermetic protection of electronic circuit” filed on 25 Oct. 1994.        “Device for the hermetic encapsulation of a component that must be protected against all stresses” filed on Sep. 11, 2001 under No: 01 14543.        
As shown in FIG. 3, certain BGA package manufacturers use a protective metal deposit 5 on five faces of the electronic module. Of course, the sixth face with the connection balls 4 does not receive this deposit in order to avoid short-circuiting the connection balls. The package, having received this type of metal, hence inorganic, protection, is hermetic on its five protected faces but not on the sixth; moisture therefore penetrates via the sixth face (the PCB face), which is not protected as illustrated in FIG. 4a. The diffusion P1 of gaseous H2O is slower by virtue of the protection and the internal pressure P3 takes longer to be reached but the effects of the penetration of H2O into the module are more significant at the level of the PCB since the pressure P3 is exerted primarily on this sixth face, which then exhibits a leakage rate that is substantially higher than the five other faces. FIG. 4b shows a view of the possible deformation 6 (shown in dashed lines) of the PCB face with the connection balls 4.
One solution for ensuring the hermeticity of ball grid array packages is to deposit a hermetic layer on the six faces of the package. Protecting the sixth face is much more difficult since it contains the external connections: balls, pads, etc. (QFN packages). This solution consists in depositing a hermetic layer on this sixth face while avoiding the balls during the deposition of this insulating layer. This is not easy for the following reasons:                A) A mask or a local stripping operation is required.        B) In the case of the mask, there is a risk that the thin inorganic layer surrounding the ball does not form a perfect seal with the ball, especially after the remelting thereof when soldering. A gap may exist and negatively effect the hermeticity of the assembly.        C) The equipment needed for this additional deposition operation in order to make the package hermetic, such as a plasma torch, is not generally used by manufacturers of electronic modules on their automated protection production lines.        